Direct drive minibot - output diameter?

[dsmoker]

Originally Posted by JVN

Obviously after the first 2 weeks of competition the secrets are all out.

If you're going to use a design that is "heavily inspired" from one you saw on the field, at least take the time to iterate the design and figure out the details yourself.

You can make this into a positive design experience for your team with some methodical experimentation...

Wow. Harsh. We asked one specific question to confirm some of our preliminary calculations. The students never asked for anyone's complete minibot design, nor do I anticipate that their final design will be "heavily inspired" by any one particular minibot that they saw perform at WPI. They simply felt that, after seeing all of the minibots in action, they wanted to try their hand at a direct-drive system to avoid the mechanical loss that naturally occurs through gearing. The drive train is but one aspect, obviously, of minibot design. As for the output diameter, the students have been experimenting with diameters ranging from 1/4" to 3/8". They simply wanted to hear what others' experiences had been in order to confirm or dispel their own conclusions. I posted their question for them because I thought that that's exactly what this forum was for.

For those who offered constructive advice on this thread, thank you!

[boomergeek]

Originally Posted by dsmoker

Wow. Harsh. We asked one specific question to confirm some of our preliminary calculations. The students never asked for anyone's complete minibot design, nor do I anticipate that their final design will be "heavily inspired" by any one particular minibot that they saw perform at WPI. They simply felt that, after seeing all of the minibots in action, they wanted to try their hand at a direct-drive system to avoid the mechanical loss that naturally occurs through gearing. The drive train is but one aspect, obviously, of minibot design. As for the output diameter, the students have been experimenting with diameters ranging from 1/4" to 3/8". They simply wanted to hear what others' experiences had been in order to confirm or dispel their own conclusions. I posted their question for them because I thought that that's exactly what this forum was for.

For those who offered constructive advice on this thread, thank you!

Let's try to assume John was not intentionally implying anything negative in the way he responded. After all, he works for a robotics company that wants everyone to use their products- so why would he ever imply anything negative about any of his customers? I'll have to assume we must be misreading anything negative.

I offer thoughts and encouragements in order to get teams to try things. Anyone who has tried to build a direct drive minibot knows that significant amounts of trials and experimentation are necessary to be successful.
Does every team need to reinvent the physics description on their own?

I think the minibot challenge this year dramatically increased the amount of time and energy that teams would spend on engineering.
I know that is true for our team. So overall, it is good for the primary goal of FIRST. It causes the most innovative teams to lose some of their advantage over the "incrementing" teams as the season wears on.
But to me, it's a no-brainer which has better impact on getting more students working on engineering problems for longer periods of time.

[roystur44]

Here's a tip for those of you without a lathe.

Take the Tetrix wheel hub, remove the set screw and drill the center out to .248" Now the wheel hub will fit right over the pinion gear. Put the locating flange of the wheel hub on the motor side. Add a little Loctite and tighten down the set screw. Now take the axle hub and drill out two of the 6-32 threaded holes(opposite). now you can take two of the allen head screws in the kit and attach the axle hub to the wheel hub. Now you have a nice coupler to attach the axle. Look in the Grainger catalog at shoulder bolts and find one that will fit your specs.

Roy

[scottydoh]

This is what 810 ended up with. Press fit onto the brass spur gear plus a set screw. The actual "drive" part of the shaft measures .287, which happens to be a perfect fit for the surgical tubing we're using.

[philso]

We did something similar to what Roy's team did. Instead of drilling out the motor shaft hub, we hooked the motor up to a bench DC supply and held a file against the pinion gear until it fit in the motor shaft hub.

We put two of the motor shaft hubs back-to-back. We were able to find a pair of holes where the threads on the two hubs for the 6-32 screws matched up without binding on the second hub.

You can then use that to hold pieces of the gear box you have cast aside.

Phil

[Ether]

Originally Posted by dsmoker

As for the output diameter, the students have been experimenting with diameters ranging from 1/4" to 3/8".

For sure, experimenting is good! But I think one example of what John might have had in mind is has anyone taught the students how to read the motor curve for the Tetrix motor and do the calculations? It's not that hard to do and can be very rewarding and inspiring for a student.

[DonRotolo]

Originally Posted by pfreivald

There are three lathes in the shop in which we work, but we aren't allowed to use any of them due to

OK, so is ANYone allowed to use the lathe(s)? If yes, have HER (or him) drill the hole for you. end of story.

Originally Posted by dsmoker

Wow. Harsh.

Not really. Just a reminder that copying is good, but learning and making it even better is, well, better. That's Engineering.

[pfreivald]

Originally Posted by DonRotolo

OK, so is ANYone allowed to use the lathe(s)? If yes, have HER (or him) drill the hole for you. end of story.

Nope. No one on the team... And the tech teachers are singularly uncooperative. (We're working on a way to end-around them, but it's taking longer than I'd like.)

And for the record, a sub-2s minibot doesn't require direct-drive with a tiny output shaft. Our custom wheels are over an inch in diameter, and we're still using one stage from the gearbox, and with deployment we're at 1.8s.

[Ether]

Originally Posted by pfreivald

Our custom wheels are over an inch in diameter, and we're still using one stage from the gearbox, and with deployment we're at 1.8s.

May I ask, what is your gear ratio and how much does the bot weigh? I assume you are using 2 motors?

[pfreivald]

Originally Posted by Ether

May I ask, what is your gear ratio and how much does the bot weigh? I assume you are using 2 motors?

Two motors, gear ratio out of the modified box is 5:1, and the weight I am not 100% sure of, but she's not much more than motors, mounts (with lightening holes drilled), two switches and some polycarb (more lightening holes).

Attached is an Inventor drawing. Ain't she cute?

[Ether]

Originally Posted by pfreivald

Attached is an Inventor drawing. Ain't she cute?

For sure. But how do you deploy it? Is it a press-fit onto the pole, or is it articulated?

[pfreivald]

The polycarb acts as a spring to space out the wheels so they fit around the pole. It's held in place with a bracket on the HOSTBOT which is punched out by a plunger on the MINIBOT when it hits the pole (this plunger also turns on the wheels, and another one turns them back off when it hits the plate).

So the MINIBOT is 'squeezed open' and held in place by a post on the deployment mechanism, then snaps in place and turns itself on using the HOSTBOT-imparted lateral kinetic energy to pop the plunger. It then drives off of the post.

Edit: Attached is a closeup shot (a bit blurry) where you can see the wheels and plunger, as well as an Inventor drawing of our deployment mechanism. (The carriage is deliberately a bit massive. This is not a subtle device!)

[JVN]

Originally Posted by dsmoker

Wow. Harsh. They simply wanted to hear what others' experiences had been in order to confirm or dispel their own conclusions.

Glad to hear. I hope everyone else is following the same order of operations. I suspect this isn't true. Kudos to you and your team.

-John

[Tom Bottiglieri]

We did some calculations to find what our desired shaft size was, built a few shafts with diameters a bit over and under that number and tried them all. It turns out in our particular case the base shaft diameter isn't as important as making sure there isn't too much wear on the tread. After it's put it about 10-20 times our tread material becomes noticeably worn and we see drastic changes in the speed.

[jacob9706]

Does anyone know the rpm of the tetroxide motor without the gear box?